1. Field of the Invention
The present invention relates to an image processing apparatus using an enhancer for enhancing predetermined frequency components of an image signal, particularly high frequency components, to improve the quality of the image.
2. Description of the Related Art
In a display for displaying an image in accordance with an input image signal, so-called “enhancement” for enhancing the high frequency components of the image signal (hereinafter referred to as the “high band components”) is effective for improving the sharpness of the image. A circuit that enhances the high band components of a signal in this way is generally referred to as an “enhancer”. In a display or another image display device, enhancement of the image signal before display improves the sharpness of the image and enables realization of a crisp image quality.
Ordinarily an enhancer is realized by a high-pass filter. By extracting high band components of the image signal with a high-pass filter and by adding the extracted high band components to the original image signal, an image signal enhanced in high band components is obtained.
FIG. 13 shows an example of the configuration of a generally used enhancer. As illustrated, the enhancer is constituted by a high-pass filter 10, a nonlinear processing circuit 20, an adder circuit 30, and a limiter 40. The high-pass filter 10 is constituted by delay circuits 11 and 12 and an adder circuit 13.
In the high-pass filter 10, the delay circuits 11 and 12 delay the input signal Sin by one or two sampling periods (Ts or 2Ts) and output the delayed signals. The adder circuit 13, which has three input terminals (taps) T1, T2, and T3, adds the input signals from the taps weighted by predetermined coefficients. By controlling the coefficients of the adder circuit 13, it is possible to control the type and characteristic of the filter.
Here, if the input signal Sin is expressed as a function of time s(t), the input signals of the taps T1, T2, and T3 can be expressed as s(t), s(t−ΔT), and s(t−2ΔT), respectively. Note that here, ΔT=Ts or ΔT=2Ts. If the coefficients of the adder circuit 13 are w1, w2, and w3, the output signal so(t) of the adder circuit 13 will be given as follows:so(t)=w1s(t)+w2s(t−ΔT)+w3s(t−2ΔT)  (1)
The nonlinear processing circuit 20 performs nonlinear processing on the output signal so(t) of the adder circuit 13, then outputs the result of processing to the adder circuit 30.
The adder circuit 30 adds the output signal S2 of the delay circuit 11 and the output signal of the nonlinear processing circuit 20, then output the result of addition to the limiter 40.
The limiter 40 limits the amplitude level of the input signal to below a predetermined threshold value.
By appropriately setting the coefficients w1, w2, and w3 of the adder circuit 13, high-pass filtering of the input signal s(t) can be achieved. Therefore, the circuit shown in FIG. 13 operates as an enhancer.
An example of the coefficients for realizing the characteristic of the high-pass filter is, for example, −¼, ½, −¼. That is, in the adder circuit 13, the coefficients provided to the taps are as shown in FIG. 14. The characteristic of the high-pass filter obtained by these coefficients is shown in FIG. 15.
Further, when the delay time of the delay circuits 11 and 12 is 2Ts, the coefficients in the case of one sampling correspond to −¼, 0, ½, 0, −¼. The characteristic of the high-pass filter obtained by these coefficients is shown in FIG. 16.
Note that in FIGS. 15 and 16, the abscissa represents the frequency, while the ordinate represents the gain of the filter. On the abscissa, the frequency is plotted in units of the sampling frequency fs. As illustrated, the maximum frequency is 0.5 fs, that is, the maximum frequency determined by the sampling theorem.
By using a high-pass filter having the frequency characteristic shown in FIGS. 15 and 16, an enhancer that enhances the high band components of the input signal Sin can be realized.
However, in the enhancers of the above related art, the frequency band being enhanced is only part of the effective frequency band of the input signal.
FIG. 17 is a graph showing the frequency components of for example a high definition signal HD having a high resolution. Since the sampling frequency fs of the HD signal is 74.25 MHz, as shown in FIG. 17, frequency components ranging up to 37.125 MHz are able to be plotted. However, the HD signal itself has almost no frequency components higher than 30 MHz, so the enhancement effect of the enhancer using the filter coefficients −¼, ½, −¼ is limited.
On the other hand, when using the filter coefficients −¼, 0, ½, 0, −¼, the frequency components in the intermediate frequency band are enhanced, while the enhancement effect for the high band components, for example, the frequency components close to 30 MHz, is poor. Furthermore, when using the coefficients described above, there is a drawback that ringing at the edge occurs in the signal passed through the high-pass filter.
FIGS. 18 and 19 show sampling values of the input signal, the filter coefficients, and the output signal of the enhancer. FIG. 18 shows a case of using −¼, ½, −¼ as the filter coefficients, while FIG. 19 shows a case of using −¼, 0, ½, 0, −¼ as the filter coefficients.
As illustrated, ringing occurs in the output signal in the case of using −¼, 0, ½, 0, −¼ as the filter coefficients.